Acrylics paints have been around since the 1960's - for a fascinating overview of their impact do read Philip Ball's The Invention of Colour. It is a fast drying paint containing pigment suspension in acrylic polymer emulsion and can be use thickly (impasto) like oils, or in thin washes like watercolour. Rather than using solvents, it is non toxic (although be aware of some pigments) non smelly and can be mixed with water or medium/gels and the brushes cleaned with water. However it is waterproof once dried so while it can be overpainted without disturbing the underlying layers it cannot be removed by re-wetting the paint. Do wear old clothes when painting!
PS The post after that will be on tools of the trade including painting knives and brushes. Several of you have already spotted my preference for Liquitex 'Freestyle' paddle brushes. I'll be telling you why later.